Humidity correction prescribed in the existing IEC standard is based on the experimental results reported in 1950's : that is, flashover voltage of insulators increases with absolute humidity. A possibility is also pointed out that the existing correction is not adequate because it occasionally results in excessive correction and causes problems in commercial tests.Under these circumstances, effect of absolute humidity on 50% flashover voltage of various ceramic and a composite insulators was studied under natural conditions by applying ac, lightning or switching impulse or dc voltage. Hydrophobicity on the surface of composite insulator surface was also studied. The main results obtained are as follows :(1) The dates of experiments were selected according to the change of absolute humidity, which was recorded at the test site over three years.(2) For most of insulators, the existing IEC humidity correction seems to be proper regardless of kinds of applied voltage.(3) Change of humidity correction seems necessary for pin type and unit suspension disk insulators for positive and negative lightning impulse flashover voltage tests.(4) Water droplets on silicone rubber vibrate and stick to each other under voltage application, which results in lower flashober voltage.(5) Flashover voltage on contaminated silicone rubber increases gradually with time lapse after contamination. This phenomenon may be attributed to the recovery of hydrophobicity due to migration of low molecular weight silicone oil from the bulk to the surface.